Ferromagnetic thin film alloy



Nov. 19, 1968 B. L. FLUR FERROMAGNETIC THIN FILM ALLOY 5 Sheets-Sheet 1Original Filed Dec. 25, 1964 FIG. 1

INV ENTOR BARRY L FLUR BY fizz/a ATTORNEY Nov. 19, 1968 B. L. FLURFERROMAGNETIC THIN FILM ALLOY 5 Sheets-Sheet 2 Original Filed Dec. 23,1964 PULSE PROGRAM WRITE 1 I READ i WRITE 0 READ 0 WORD BIT

SENSE FIG. 4

FIG.

Nov. 19, 1968 s. FLUR 3,411,950

FERROMAGNETIC THIN FILM ALLOY Original Filed Dec. 23, 1964 3Sheets-Sheet 5 Hxo OERSTEDS TEMPERATURE C TEMPERATURE C OERSTEDSTEMPERATURE C United States Patent 3,411,960 FERROMAGNETIC THIN FILMALLOY Barry L. Flur, Poughkeepsie, N.Y., assignor to InternationalBusiness Machines Corporation, Arrnonk, N.Y., a corporation of New YorkOriginal application Dec. 23, 1964, Ser. No. 420,754, now Patent No.3,303,117, dated Feb. 7, 1967. Divided and this application Oct. 20,1966, Ser. No. 600,309

1 Claim. (Cl. 14831.55)

ABSTRACT OF THE DISCLOSURE A ferromagnetic thin film alloy of the typefinding adaptation as a storage and switching device consisting of 2 to6% by weight molybdenum, from about 14 to 19% by weight iron, with thebalance nickel, characterized by an anisotropy field of up to about 0.5oersted and a coercive force of about 0.5 oersted.

This patent application is a divisional application of US. patentapplication 420,754, filed Dec. 23, 1964, now US. Patent No. 3,303,117,granted Feb. 7, 1967.

A concerted effort is presently under Way, by both the scientific andacademic community, to study and to develop ferromagnetic thin films foradaptation as parametrons, delay lines, logic devices and storageelements for computers. What has given impetus to this effort is thediscovery by M. J. Blois, IL, in 1955, that ferromagnetic thin films of80:20 (by weight) nickel-iron, when evaporated in the presence of amagnetic field, exhibit uniaxial anisotropy. With uniaxial anisotropy,an easy axis of magnetization is furnished which is parallel to thedirection of the externally applied field, along which are found twostable states corresponding to positive and negative remanence. Also,these ferromagnetic thin films tend to favor a domain structure thatallows rapid rotation of the magnetic remanence from one stable state tothe other. Potentially, both engineering and economical advantages areoffered over present storage and switching devices used in dataprocessing and computer machines.

Storage or switching of intelligence is achieved by magnetizing aparticular element or bit, in an array of such elements, into either oneor the other of its stable states. Rotation of the magnetizationremanence takes place, upon application of the required switchingfields, from one stable state to the other, in short periods of time, inthe order of nanoseconds Characteristics such as these lend themselvesto the applications as heretofore described.

Various techniques are available for preparing ferromagnetic thin filmdevices that exhibit uniaxial anisotropy. These include: vacuumdeposition, electroplating, chemical reduction, pyrolytic methods, andcathode sputtering. The first two of these methods have received wideattention in the literature. Chemical reduction or electroless platinginvolves the reduction of metal salts such as those of nickel, iron, andcobalt, with hypophosphite on an active or catalytic surface. Thepyrolytic method, a process which has not attracted the interest such asthat focused on the others, entails thermally decomposing an appropriatemetal-organic compound, such as the mixtures of the nickel and ironcarbonyls.

Now, as to the last of these processes, cathode sputtering is a processin which atoms are ejected from the surface of a material through theimpact of ions or of atoms. Commonly, the procedure for causing ions tostrike a material and eject atoms, employs an enclosed chamber,maintained at a pressure from about 1() to about 10 tort or higher, inwhich are mounted two plates in parallelspaced relation. A DC source iscoupled to the plates to furnish a potential of several thousand volts.The material that is sputtered or is mounted on one plate, that is, themedium giving up its atoms, is generally designated the target, and thesubstrate, the surface upon which the ejected atoms are collected, ispositioned on the other plate. The potential applied between the platesproduces positive ions in the glow discharge between the plates, and thepositive ions are accelerated toward the target, ejecting atoms ormolecules therefrom. Since almost the entire applied voltage in a glowdischarge is dropped across the ion sheath that surrounds the target,the target is under steady bombardment by high-energy ions, the impactof which impels atoms of the target material to leave its surface, whichatoms flow toward the substrate surface upon which the film is formed.

Although success has been achieved with some of these heretoforementioned techniques, the implementation of a thin film ferromagneticdevice, where the film is a product of a cathode sputtering process andyielding the magnetic characteristics and economic advantages thattheory affords, was a substantial problem until the advent of theprocess which is the subject of copending patent application of Leon I.Maissel et al., Ser. No. 402,800, filed Oct. 9, 1964, now US. Patent3,303,116 granted Feb. 7, 1967, which patent application is assigned tothe assignee of the instant application.

That process of Leon I. Maissel et al. provides reproducible magneticalloy thin films from the plasma environment of a glow dischargeprocess. That process enables control over structure and compositionthat was heretofore not available from prior art processes, therebyfurnishing magnetic properties such that a range of the same may bepredictably built into the film. That process utilizes thin foils orsheets of ferromagnetic material which are subjected to ionicbombardment and the products of the bombardment collected on a substratewhile the condensing atoms or molecules are subjected to a suitablebias. Thus a cathode sputtering process is provided for producingferromagnetic thin films which is both economically and scientificallycompetitive with magnetic thin films produced by other processes.

It appears desirable that a ferromagnetic thin film, prepared forcomputer utilization, have low anisotropy fields, to permit rapidswitching at low drive currents and, further, as based on experimentaland analytical studies, have a low dispersion of the easy axis toprovide disturb-insensitive films. It is in the anticipation ofobtaining such films that the industry has focused its attention on thePermalloy type film. That film, Permalloy, normally contains from 55% tonickel with the balance iron and, in bulk, exhibits high permeability,low coercive force, low magnetostriction, and low anisotropy. But,although the magnetostriction and anisotropy are sufficiently low forsome computer functions, investigators in the art have found that thezero magnetostriction alloy (81:19 nickeliron) is not the samecomposition that exhibits the lowest anisotropy (78:22 nickel-iron): itappears that the crystalline anisotropy is an important contributor tothe dispersion of the easy axis and, in some instances, may contributeto the total anisotropy as well. As a result of the increased stress nowbeing placed on computers capable of operating at higher speeds, therelatively high crystalline anisotropy of the Permalloy type film hascaused attention to be directed to other ferromagnetic film materialswhich, from their properties in bulk, appear to be more suitable forcomputer use.

Why anisotropy and magnetostriction are important parameters, forevaluating the suitability of a ferromagnetic thin film material forcomputer applications, is readily gained from the discussions in theliterature. But, a brief review of the import of these phenomenon maylend to the appreciation of the present contribution. Of course, it isto be realized that many of the fundamentals and details of thediscussion that is to follow are, by necessity, greatly simplified.Ferromagnetism, it is generally accepted, is directly related to thespin of the electrons in material. One may think of each electronbehaving very much like a bar magnet which, in a magnetic field, canalign itself either with the field or against it according to its spin.To magnetize a material, more of its electrons must spin one way thanthe other so that an excess of elementary magnets point in onedirection. The regions in the material in which electron spins areparallel are designated domains, and these domains are representative ofthe lowest energy configuration for the assembly of elementary magnets.When an external magnetic field is applied, the magnetic energy ofdomains oriented in the direction of the field is lowered; the magneticenergy of the domains oriented against the field is raised.

To important parameters related to the magnetic state of material arecrystalline anisotropy and magnetostriction. Although both of thesephenomenon are not totally divorced one from the other, the discussionthat is presented hereafter shall treat these phenomenon, for purposesof clarity and ease of understanding, as independent one from the other.

The crystalline anisotropy energy, or, as is sometimes called, themagnetocrystalline energy, of a ferromagnetic crystal acts in such a waythat the magnetization tends to be directed along certain definitecrystallographic axes which, accordingly, are called directions of easymagnetization. The directions along which it is most dilficult tomagnetize the material are called the hard directions. It is theavailability of well defined easy and hard directions that has attractedthe attention to ferromagnetic thin films: the anisotropy provides thebistate behavior which is sought in the film. But, as heretoforementioned, the crystalline anisotropy is an important contributor to thedispersion of the easy axis, as well as, possibly, contributing to theanisotropy energy as a whole. High values of crystalline anisotropy leadto increased power requirement, specifically, in the Word drivers.

Also, influencing the total anisotropy of the material is magnetoelasticenergy which is that part of the energy which arises from theinteraction between the magnetization and the mechanical strain of thematerial. It is known from theoretical and experimental considerationsthat there is aclose physical relationship which exists betweencrystalline anisotropy and the magnetostriction phenomena in a magneticmaterial. Thus it is readily appreciated that it is most advantageous,in order to achieve higher speeds of operation, in a computer, toprovide ferromagnetic thin films which exhibit low anisotropy fields aswell as a minimum of magnetostriction effects.

Ternary ferromagnetic films of nickel-iron-molybdenum appear, from theirproperties in bulk, to offer these desirable features. From experiencewith bulk alloys of nickel-iron-molybdenum, particularly 79-nickel,17-iron, 4-molybdenum, it was expected that essentially zeromagnetostriction and low anisotropy fields would be exhibited by thinfilms of that composition, since bulk material in thicknesses roll tomil thick foil and appropriately heat treated exhibit square hysteresisloops along with the other sought magnetic properties. Although theoryand experience would indicate that ternary alloys ofnickei-iron-molybdenum should provide ferromagnetic film materials thatexhibit low magnetostriction and low crystalline anisotropy inconjunction with other desired properties for computer applications, therealization of the ternary ferromagnetic thin film of this compositionis still wanting.

Now, further improvements to the heretofore mentioned Maissel et al.process have been discovered that now makes it possible to provide aternary ferromagnetic film of nickel-iron-molybdenum that exhibits thedesired characteristics sought for computer applications. What has beenfound is that the cathodic sputtering of a ternary ferromagnetic thinfilm of the nickel-iron-molybdenum composition yields a ferromagneticthin film with crystalline anisotropy energizes lower than thatpresently available with the Permalloy type films. The ternaryferromagnetic thin film of nickel-iron-molybdenum, which is a product ofthe cathode sputtering process, now makes it possible to decrease thepower requirements for operating computers, thereby increasing the speedof operation with nearly perfect reliability.

Prior to the preparation of the present ferromagnetic thin film ofnickel-iron-molybdenum with the desirable properties, various procedureshave been attempted to accomplish the same. For example, others haveemployed cathode sputtering for producing the ternary ferromagnetic thinfilm of nickel-iron-molybdenum, but, in those instances where cathodesputtering was used to sputter that alloy, the sputtering was done atlow pressures, which requires the use of an externally supported (hotfilament) glow discharge to enable the sputtering to take place. Such aprocess and the product resulting therefrom has many shortcomings anddeficiencies compared to a self sustained glow discharge sputteringprocess and product, such as that which is the subject of the presentinvention. A low pressure cathode sputtering process and product asdescribed in the prior art are not commercially nor scientificallycompetitive with a self sustained glow sputtering process for producingferromagnetic thin films.

Sputtering in a self sustained glow discharge begins to fall off atpressures below about 20 microns because of the rapid decrease in thedensity of the ions with decreasing pressure, but the increased energyof the ions at low pressures and the longer mean free path of thesputtered atoms do not compensate for the reduced ion density. To enablesputtering at reasonable rates at low pressure, the supply of ionizingelectrons or ions must be increased or the ionization deficiency of theavailable electrons greatly improved. Although this can be done to someextent, it complicates the sputtering process, particularly the verysevere problem of obtaining uniform deposits over large areas. A processthat requires elaborate procedures for operations such as the lowpressure sputtering process and yet does not yield uniform deposits isneither commercially nor scientifically competitive with ferromagneticthin film presently available from the heretofore mentioned prior artprocesses.

Of more recent date, attempts have been made to form ferromagnetic thinfilms of nickel-iron-molybdenum by other techniques in order to takeadvantage of the properties afforded by theory. Variable ratiosequential and simultaneous two-source evaporation has been employed,but the magnetic characteristics reported for these evaporatednickel-iron-molybdenum ferromagnetic thin films are not sufiicientlyattractive to warrant their implementation for computer application.

While investigations have been carried on for ways of preparingnickel-iron-molybdenum films and cathode sputtering procedures have beenutilized in this endeavor, no one has provided a ferromagnetic thin filmof nickeliron-molybdenum that is sufiiciently attractive to capture theattention of the computer industry. Nor has anyone proposed a generalapproach for preparing such films, which approach promisesnickel-iron-molybdenum ferromagnetic thin films with the desiredproperties and which ferromagnetic thin film is also producible atcommercially accepted yields. Consequently, a desirable replacement forthe conventional nickel-iron films of the Blois type have not beenrealized, nor has the inviting advantages, inherent to cathodesputtering, been exploited.

The present invention is based on a discovery that anickel-iron-molybdenum ferromagnetic thin film is available thatexhibits low anisotropy, low coercive force, and desirable values ofdispersion, which ferromagnetic thin film is adaptable for computerapplications. This is achieved with a cathode sputtering process inwhich the deleterious attributes and disadvantages of the heretoforedescribed prior art attempts have been eliminated. In obtaining thefilm, a target which is a thin foil sheet of the ferromagnetic material,that is, an alloy of nickel, iron and molybdenum, is subjected to ionicbombardment in a cathode sputtering system. The atoms of target materialejected by the impact of the ions are collected on a substrate, on thesurface of which temperature gradients are minimized and an essentiallyuniform profile of the same provided. The ejected atoms, whilecondensing on the substrate, are subjected to a suitable bias. Theresulting ferromagnetic films of nickel-iron-molybdenum exhibit uniformproperties on the surface thereof and coercive forces, anistropy fields,and magnetostrictive characteristics which are sufficiently attractivefor computer applications.

The presence of these attractive magnetic properties indicates thatimpurity contamination is avoided or has been reduced to levels that arenot detrimental to the film properties. In addition, selectively induceddirections of magnetization are well defined indicating that fielddistortion does not take place or is not of sufficient magnitude toinfluence the anisotropy. Further, in accordance with the presentinvention, the tendency to trap and retain large amounts of impuritiesduring deposition, a problem usually encountered in vacuum depositionprocedures, is avoided. That problem is usually rather severe forsputtering in comparison to other deposition procedures, because of thebombardment of fixtures and inner surfaces of the vacuum chamber byhigh-energy ions of the sputtering gas, which produces reactive gases,not easily detected and, further, because ionized species present duringthe sputtering are generally more reactive than their neutralcounterparts. The selective application of a negative bias (relative tothe anode) to the film, forming from the ejected atoms on the surface,yields the long sought ferromagnetic thin film ofnickel-iron-molybdenum.

Accordingly, it is a principal object of the invention to provide aferromagnetic thin film of nickel-iron-molybdenum exhibiting magneticcharacteristics desirable for implementation for computer applications.

It is a further object of this invention to provide an improved processfor cathodically sputtering ferromagnetic thin films containingnickel-iron-molybdenum.

It is yet another object of this invention to provide an improvedprocess for cathodically sputtering ferromagnetic thin films ofnickel'iron-molybdenum, which films exhibit uniformity of magneticproperties.

It is yet another object of this invention to provide an economicallyand commercially feasible process for cathodically sputteringferromagnetic thin films of nickeliron-molybdenum for adaptation in dataprocessing and computer machines.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention as illustrated inthe accompanying drawings.

In the drawings:

FIG. 1 is a schematic representation of the cathode sputtering apparatusutilized in the preparation of a magnetic thin film.

FIG. 2 is a schematic representation of a storage bit cell.

FIG. 3 is a typical pulse program utilized in the operation of thestorage device of FIG. 2.

FIG. 4 is a schematic representation of the microscopic variance of themagnetization vector from the intended easy direction of magnetizationto illustrate skew and dispersion.

FIG. 5 is a schematic illustration of the clip used to bias the filmcondensing on a nonmetallic substrate.

FIG. 6 is a schematic illustration of the anode assembly of FIG. 1.

FIG. 7 is a plot of anisotropy (H in oersteds versus temperature in C.

FIG. 8 is a plot of dispersion [5 in degrees versus temperature in C.

FIG. 9 is a plot of coercivity (H in oersteds versus temperature in C.

Before turning to a more specific discussion of the present invention,several magnetic parameters are briefly reviewed. In particular coerciveforce (H anisotropy filed (H and dispersion (B) are of significance inevaluating the anisotropy properties of ferromagnetic thin films andtheir function in computers. These terms are well known in the art andare widely described in the literature, such as, for example in thearticle by H. I. Kump, T he Anisotropy Fields in Angular Dispersion ofPermalloy Films 1963, Proceedings of the International Conference onNon-Linear Magnetics, Article 12-5. But, to facilitate the discussion athand, the terminology is briefly reviewed:

H coercive force is a measure of the easy direction field necessary tostart a domain wall in motion, a threshold for wall motion switching.

H -anisotropy field may be thought of as the force required to rotatethe magnetization from its preferred direction of magnetization to thehard direction and, H is the anisotropy field as viewed on a microscopicscale.

fiDispersion is conveniently defined with reference to FIG. 5 whichshows a section of a magnetic thin film, as comprising the aggregate ofmicroscopic magnetic regions n. Associated with each of the microscopicmagnetic regions 11 is a magnetization vector 11. Under idealconditions, each of the magnetization vectors 11, related to amicroscopic magnetic region n, is parallel one to the other with thevector summation thereof yielding the intended easy direction ofmagnetization depicted as arrow 300. But, owing to various imperfectionsand fabrication difficulties, some of which are hereafter discussed, theintended easy direction of magnetization, arrow 300, is not achieved.The mathematical mean for the magnetization vectors n gives rise to amean easy direction of magnetization designated arrow 302, and the anglea, between the intended easy direction, arrow 300, and the mean easydirection, arrow 302, is commonly referred to as skew. Now, the angle inwhich we find of the microscopic magnetization vectors n of themicroscopic magnetic regions n is dispersion, and that angle is graphically illustrated in FIG. 4 as the angle between the mean easy axis,arrow 302, and the boundary line, arrow 304, which includes 90% of thedeviation of the magnetization vector u from the intended easy axis ofmagnetization arrow 300. Measurement of dispersion is similar to thatdiscussed ir: the article by T. C. Crowther, entitled Techniques forMeasuring the Angular Dispersion of the Easy Axis of Magnetic Film,Group Report #51-2, M.I.T. Lincoln Lab, Lexington, Mass. (1959).

Now, speaking generally as to the conditions heretofore described,regarding the cathode sputtering of a ferromagnetic thin film, referenceis made to FIG. 1, for convenience, which schematically illustrates thegeneral type of apparatus, depicted as numeral 10, utilized in thepractice of the invention. Apparatus 10 includes a first electrode 2,but cathode assembly, formed to also function as a heat sink, in thatthe lower portion 2a is added to the mass of the upper portion 2b. Rapidwithdrawal of thermal energy from the face of the electrode isfacilitated by the high conductivity of 2a and 2b and the largeradiating surface dissipating heat to the cooling shield 8, hereafterdescribed. Bonded to the surface of cathode assembly 2 is a thin foil offerromagnetic material 4, the target. Coupled to the cathode assembly 2is the negative lead 6 of a voltage source (not shown). A shield 8,having cooling coils 7 about its periphery, is positioned around cathodeassembly 2, within the Crookes dark space distance from the cathode. TheCrookes dark space is a well known term in the art and is described inVacuum Deposition of Thin Films by L. Holland, pp. 80-82 (1961 ed.).

Below cathode assembly 2 is placed, in substantially parallel spacedrelation thereto, anode assembly 14, the schematic of which is shown inFIG. 6 including heating coils 16 for providing a uniform temperatureprofile on carrier portion 18 which is held by flanges 20. On thesurface of the anode carrier portion 18 is support 22, which ispreferably glass, which serves to prevent substrate 24 from makingcontact with thee anode. In the particular apparatus utilized, a spacingof 2.5 centimeters is maintained between cathode and substrate, but anyconvenient spacing is permissible, providing it is maintained at adistance greater than the dark space distance. Anode assembly 14 isgrounded via a lead 26 while the film that condenses on substrate 24 isbiased as required via lead 28 coupled to the support 22.

Positioned between cathode assembly 2 and anode assembly 14 is rotatableshutter 30 which is placed between cathode 2 and anode 14 during thepresputtering cleaning of the cathode. This is done to assure theremoval of all contaminants from the surface of the ferromagnetictarget. Once the precleaning step is performed, shaft 32 rotates shutter30 away from its station between cathode 2 and anode assembly 14 toleave the ferromagnetic target 4 facing anode assembly 14.

Enclosing the electrodes is bell jar 34 which, in the particulararrangement, has a diameter of about 18 inches Bell jar 34 rests on basemember 36 which contains two ports 38 and 40. The first port 38 is aninlet for a suitable gas via conduit 38a and control valve 38b. Argon,for example, furnishes the necessary ionized particles for bombardingthe surface of the ferromagnetic foil. The second port 40 serves toconnect a second conduit 40a which, in turn, is controlled by a valve40b, and, is coupled to a vacuum pump 42. It is usual to maintain theenvironment within the bell jar at a pressure in the range between to10* torr. Two coils 44a and 4417 are mounted externally in bell jar 34to provide a uniform magnetic field in the vicinity of the glowdischarge, the coils being arranged to induce a magnetic field parallelat the surface of the substrate. To maintain a uniform field over thesubstrate surface requires relatively large coils.

By way of example, a vacuum melted and rolled 12.5 centimeter square 79Ni-17 Fe-4 Mo sheet is bonded to the surface of the cathode assembly 2and the vacuum system pumped down to less than 1 10 torr. Substrate 24is mounted on support 22. Desirable materials for substrates are metalssuch as silver, copper, aluminum, or the like, or a nonmetallic, such asglass. Where metal is used as the substrate, lead 28 for biasing thefilm need only make contact with the bottom surface of the metalsubstrate. But, where a nonmetallic such as glass is the substrate, aclip is used as depicted in FIG. 5, lead 28' is passed through apertures23' provided in glass support 22' and coupled to the base of clip 27'which encases the bottom and side surfaces of the glass substrate. Theelbows 29 of clip 27' contact the periphery of the surface upon whichthe sputtered material condenses. In those instances where it is desiredto maintain a continuous bias on the film during the sputtering process,a land 21, which is a thin line of metal of the same composition as thetarget, is predeposited on the surface of the substrate by any of theconventional techniques. Clip 27' is then coupled to the predepositedland to provide a conductive path over the substrate surface. Ashereafter explained, the bias to the film is most effective after acontinuous layer of sputtered material collects on the surface of thesubstrate. The continuous layer then serves as a conductive path to theclip 27', and dispenses with the need for predeposited land 21'. Thesubstrate is not clamped to support since this introduces stresses inthe sputtered deposit, and stressing the film affects the uniformity ofthe magnetic properties. The bias clip, heretofore described, is mountedabout the substrate to avoid the introduction of stresses in the devicearea of the film.

To provide the bombardment media for ejecting the atoms from the target,which in the example given was of a twenty mil thickness, argon isinjected through port 38 to a pressure of approximately 0.1 torr,through conduit 38a and the regulation thereof maintained by valve 38b.With shutter 30 interposed between the cathode 2 and substrate 24,target 4 is cleaned by presputtering, as discussed above, to removecontaminants from the surface thereof. Following the cathode cleanup,shaft 32 rotates shutter 30 from the intermediate position betweentarget 4 and substrate 24. Thereafter a potential of about 2000 volts,for example, is applied between the cathode and an anode at a current ofabout 110 milliamperes. Once the glow discharge is initiated, a bias ofabout volts is applied to the substrate by way of lead 28 and a magneticfield of about 25 oersteds is applied by way of coils 44a and 44b toinduce the magnetic anisotropy in the desired direction in the sputteredmaterial. The sputtering is conducted for seconds to produce a film witha thickness of about 700 Angstroms.

Heating of substrate 24, upon which the ejected atoms from target 4collect to form the ferromagnetic thin film, is important. In order toobtain the desired magnetostriction, coercive force (H anisotropy field(H and dispersion (,8) in a ferromagnetic thin film ofnickel-ironmolybdenum, the temperature gradient over the surface ofsubstrate 24 must be minimized, and, more desirably, the profilethereof, maintained uniform. The affect of substrate temperature uponthe magnetic properties is illustrated by FIGS. 7, 8 and 9 which arehereafter explained.

A ferromagnetic thin film containing 79% by weight nickel, about 17% byweight iron, and about 4% by weight molybdenum, and grown to a thicknessof about 700 Angstroms, exhibited an anisotropy field of about 0.5oersted, a coercive force of about 0.4 oersted, and a dispersion within5". There is no upper limit to the thickness of the film available fromthe process, in accordance with the present invention, save that wheredemagnetizing fields would begin to have deleterious effects on thefilm, in the particular application for which it is employed. However,for computer applications, it is desirable to maintain the thickness ofthe film below 3000 Angstroms and preferably between 700 to 1000Angstroms, Elec tron micrographs of the 700 Angstrom thick films takenby the direct replica technique at 54,000 X show a grain size between200 to 600 Angstroms with an average grain. size of about 400 Angstroms,a grain being defined as the spherical elevation in the electronmicrograph.

Ferromagnetic thin films containing molybdenum up to 6% by weight, witha nickel to iron ratio of about 4:1, are derived from the cathodesputtering process of the present invention. Ferromagnetic thin filmscontaining from about 2% to 6% by weight molybdenum, 14% to 19% byweight iron, with the balance nickel, are preferable, and optimummagnetic properties are available with a ferromagnetic thin film of thecomposition 79% by weight nickel, 17% by weight iron and 4% by weightmolybdenum. The ferromagnetic thin film alloys, in accordance with theinvention, are produced with the anisotropy fields, coercive force, anddispersion as heretofore discussed, and with essentially zeromagnetostriction. Similarly, electron micrographs indicate that thegrain size for these nickel-iron-molybdenum films is between 200 to 600Angstroms and do not reveal a preferred grain direction.

The electron micrographs, heretofore briefly mentioned, were taken by adirect replica technique. In that technique a carbon film was depositedover the surface of a ferromagnetic thin film. The carbon coated thinfilm was then immersed in a solution of hydrochloric acid and theferromagnetic film etched away to leave the carbon replica thereof. Theelectron micrograph measurements were taken from the replica at amagnification of 54,000X.

A magnetic thin film formed by the processes heretofore defined formspart of a storage matrix and one bit cell for such a matrix is shown inFIG. 2. Usually a series of these bit cells, generally depicted asnumeral 50, are arranged in rows and columns with their associatedconductors, that, is the word lines W and the common-bit sense lines BSdisposed in such a manner that the con ductors are substantially inquadrature one to the other. Bit cell 50 includes a base portion 52,which may be glass, mica, metal or the like. Where metal is used, itserves also as the ground return for the lines W and BS therebyattaining closer inductive coupling to the device. Over base 52 layer 54of chromium and layer 54' of silicon monoxide are deposited. Themultiple layers of both chromium and silicon oxide are used to reducethe surface roughness and increase adhesion. The ferromagnetic film 56is placed over layer 54; drive lines W and BS complete the device. Arrow100 in the device represents the easy direction of magnetization whichis parallel to the drive line W while arrow 200 represents the hard axiswith the drive lines BS being parallel, or in other words, transverse toarrow. Bit cell 50 is word organized with the Word lines W uponactivation, furnishing a field transverse to the easy direction ofmagnetization of sufiicient magnitude to rotate the magnetization 90from the easy axis, while bit sense line BS upon activation, produces afield parallel to the easy axis 100.

For purposes of discussion, to illustrate the operation of the cell 50,assume that the remanent magnetization representing data is orientedalong the direction of arrow 100. With a field along the drive line Wthe magnetization vector rotates away from the easy axis arrow 100toward the hard axis arrow 200. Upon activation of the drive line BSdepending upon the polarity of the applied field (note that the bit lineis activated before and deactivated after the word pulse) themagnetization vector falls either toward 100 or 100"; the state assumedupon cessation of the word pulse determines the polarity of theintelligence to be stored, that is, in binary nomenclature whether abinary 1 or a binary is stored. Sensing of this stored information isachieved with activation of the drive line W during the rise time of theword pulse.

FIG. 3 depicts a typical pulse program for energizing the drive line Wand BS a discussion of which may lend to the understanding of theoperation of bit cell 50. To store a binary 1, first a pulse of positivepolarity is aplied along the drive line W driving the storedintelligence toward the arrow 200. Were information previously stored inthe bit cell, a sense amplifier (not shown) coupled to the common-bitsense line BS would detect a signal such as that indicated under FIG.30. Following the activation of the word line with a field ofapproximately 3 H (oersteds), drive line BS is activated, having a fieldstrength of about 0.5 H (oersted), the time sequence for the activationof the pulses of both the word and bit drive lines illustrated in FIGS.3a and 3b. With a positive pulse along line BS the magnetization vectorrotates toward 200", thereby storing a binary 1. Were a binary 0desired, the drive line is activated, with a positive polarity asheretofore described, but, the polarity of the field induced along thebit drive line BS is opposite to that of the polarity induced for thestorage of the 1 resulting in the magnetization vector resting along at200'. The requirements of the bit pulses are that they are large enoughto assure complete rotation either to the right or left of 200 but smallenough not to disturb intelligence stored along adjacent bits. The wordpulse program requires that the applied fields are large enough to driveall bits toward 200 which represents the hard direction ofmagnetization. In principle there is no upper limit to its magnitude.

Other modes of operating a storage device are available, as described inthe copending patent application of Bruce I. Bertelsen et al., Ser. No.334,858, filed Dec. 31, 1963, and assigned to the assignee of theinstant application. Operation of this device is based on device 50having two additional quasistable magnetization positions in a directionorthogonal to arrow 100. In FIG. 2, as previously, arrow represents theeasy axis, but now positions 200 and 200" of arrow 200, the hard axis ofmagnetization, are utilized as additional quasistable states. Thestability of these positions, which is initially unstable, is broughtabout by mutual locking of the magnetization subzones into which thefilm decomposes after the magnetic field pulse has ended.

To operate the device with four stable states, the word pulse isactivated with the appropriate field strength to rotate themagnetization vector from the easy axis toward the hard axis. Dependingupon the polarity of the applied word pulse, the magnetization vectorassumes either position 200' or 200". To rotate the store-d informationfrom the hard axis, arrow 200, to a position along arrow 100, activationof both the word W and bit lines BS is required. Reading is performed ina similar manner, as heretofore described, for the conventional mode;the output signals are sensed upon the leading edge of the ap plied wordpulse, with the polarity of the sensed signal depicting the intelligencestored.

The manner in which the bias is applied to the ferromagnetic film duringformation depends on the substrate material upon which the ejectedtarget atom are collected. In those instances where nonmetallicsubstrates are employed, activation of the bias to the film during theentire period of collection of the ejected atoms is acceptable. But, inthose instances where the ferromagnetic thin films are cathodicallydeposited on metallic substrates substantially covered with anonmetallic such as silicon monoxide or the like, the period of delayprior to the activation of the bias to the film is required.

It is common to predeposit layers of silicon monoxide on the substratesurface since it has been found that the magnetic properties are greatlyenhanced as a result of the underlayer of the silicon monoxide. Thesilicon monoxide, it is hypothesized, reduces surface roughness andserves as a diffusion barrier between the substrate and the film. Whereincreased adhesion is sought, alternate layers of chromium and siliconmonoxide are predeposited such as illustrated by layers 54 and 54respectively of the device of FIG. 2 (see Silicon Monoxide Undercoatingfor Improvement of Magnetic Film Memory Characteristics, B. I.Bertelsen, Journal of Applied Physics, vol. 33, No. 6, pp. 2026-2030,June 1962). With a metallic substrate having a superimposed layer of anonmetallic such as the silicon monoxide, continuous application of thebias to the collecting film result in contamination of the resultingdeposit which depreciates the characteristics of the mag netic film.This is avoided by delaying the application of the bias to the substrateuntil the ferromagnetic target has undergone ionic bombardment for apredetermined period to permit the collection of a continuous layer overthe substrate. The problem is not so much a question of the magnitude ofthe initial layer thickness to which the condensed film develops priorto activating the negative bias (with respect to the anode) but towithhold the application of the bias until the deposited layer providesa continuous conductive path over the surface of the substrate.

What has been described is a ferromagnetic thin film alloy containing upto 6% by weight molybdenum, with the balance being nickel and ironessentially in the ratio of 4:1, characterized by superior uniformmagnetic properties including low anisotropy fields, low coercive force,essentially zero magnetostriction and other desirable properties forcomputer applications. Desirably, the ferromagnetic thin film alloycontains from about 2% to 6% by weight molybdenum, from about 14% to 19%by weight iron, with the balance nickel. Optimum magneticcharacteristics are available with the ferromagnetic thin film thatcontains about 4% by weight molybdenum with the balance of nickel andiron being approximately in the ratio of 4: 1. The ferromagnetic thinfilms are the product of a cathode sputtering process in which process abias is applied to the sputtered material as it collects on a substrate.Electro micrographs, of the direct replica type, taken at amagnification of 54,000X, show that these ferromagnetic thin films havea grain size between 200 to 600 angstroms and do not reveal a preferredgrain direction. As will be apparent to those skilled in the art, theillustrated arrangements for the apparatus can be readily modifiedwithout departing from the principles of the herein described invention.It will be readily appreciated that bombarding media other than argonare usable in the process in accordance with the present invention. Forexample, helium, neon, krypton, mercury and xenon are usable to providethe source of the bombarding ions.

While the invention has been particularly shown and 12 described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. A ferromagnetic cathodically sputtered thin film alloy consistingessentially of about 2% to 6% by weight molybdenum, with the balancebeing nickel and iron, with a nickel to iron ratio of about 4:1, saidalloy having a grain size of up to about 600 angstroms and furtherwherein said alloy is characterized by superior uniform magneticcharacteristics including an anisotropy field of up to 0.5 oersted, acoercive force of about 0.5 oersted and a dispersion within 5".

References Cited UNITED STATES PATENTS RICHARD O. DEAN, PrimaryExaminer.

